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Palaeogeography, Palaeoclimatology, Palaeoecology
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Belemnite taphonomy (Upper Jurassic, Western Tethys) part II: Fossil–diageneticanalysis including combined petrographic and geochemical techniques
M. Isabel Benito a,⁎, Matías Reolid b
a Dpto. Estratigrafía, Universidad Complutense de Madrid, IGEO-CSIC, 28040 Madrid, Spainb Departamento de Geología, Universidad de Jaén, Campus Las Lagunillas, 23071 Jaén, Spain
Fossil–diagenetic features were analyzed on 56 belemnite rostra from the Pozo Cañada section (External Prebetic),as well as 31 belemnite rostra from the Río Segura (Internal Prebetic), both from the Upper Oxfordian–LowerKimmeridgian. They mainly correspond to Hibolithes and, secondarily to Belemnopsis. Fossil–diagenetic processeswere analyzed in each specimen, using petrographic (conventional, cathodoluminescence, epifluorescence andscanning electron microscopy) and geochemical (elemental and stable isotopes) techniques.The most common fossil–diagenetic processes are dissolution, calcite cementation, and recrystallization ofthe apical zone and outer growth rings of belemnite rostra. These processes may appear enhanced by fractur-ing and stylolite formation. Petrographic study also reveals that the alternation of cloudy and clear concentricgrowth areas displayed by many belemnite rostra corresponds to an early diagenetic feature in origin. How-ever, an original concentric growth pattern is also observed under epifluorescent microscopy and BSEM. Thisgrowth pattern fits with changes in the Mg and S content of the rostra.Although fossil–diagenetic processes typically make specimens non-suitable for paleoenvironmental inter-pretations, microsampling of petrographically altered and non-altered areas from the same specimens,performed directly from thin sections after petrographic study, allowed us to obtain excellent geochemicalresults suitable for paleoenvironmental interpretations. These geochemical analyses moreover demonstratesthat caution should be taken if elemental analyses are used as the most significant criteria for discriminatingdiagenetically altered and non-altered belemnite samples.
Studies of belemnite rostra, focusing on carbon, oxygen and stron-tium isotopes, are common in literature. For example, interpretationsof paleoclimatic conditions during the Callovian–Early Oxfordian transi-tion and during the Oxfordian–earliest Kimmeridgian (e.g. Price andSellwood, 1994, 1997; Price and Gröcke, 2002; Wierzbowski, 2002,2004; Nunn et al., 2009; Price and Rogov, 2009; Wierzbowski et al.,2009; Alberti et al., 2011; Wierzbowski and Rogov, 2011), as well asisotopic analyses of belemnites from other Jurassic intervals (e.g.Spaeth et al., 1971; Veizer, 1974; Veizer and Fritz, 1976; Podlaha et al.,1998; Niebuhr and Joachimski, 2002; Rosales et al., 2004a,b; Dutton etal., 2007; Wierzbowski and Joachimski, 2007; Nunn and Price, 2010;Price and Nunn, 2010; Gómez and Goy, 2011; Li et al., 2012)have been based on the analysis of stable isotopes of C and O frombelemnites. Moreover, studies on belemnite rostra are also related tostrontium isotopes (e.g. Jones et al., 1994a, 1994b; Veizer et al., 1999;McArthur et al., 2000; Nieto et al., 2008). Further detailed works stemfrom the interpretation of concentric or seasonal growth rings and the
effect of diagenesis (Spaeth et al., 1971; Sælen, 1989; Sælen andKarstang, 1989; Podlaha et al., 1998; Dutton et al., 2007; McArthuret al., 2007a; Wierzbowski and Joachimski, 2009), though the interpre-tation of concentric rings remains controversial.
In general, such studies rely on cathodoluminescence, scanningelectron microscopy, and/or manganese and iron concentrations andSr/Mn ratios to determine diagenetic effects that can change the origi-nal chemistry and isotopic signal of biogenic carbonate. One exceptionis the excellent paper by Sælen (1989), describing the morphologyand diagenetic alteration of belemnite rostra based on conventionalmi-croscopy, cathodoluminescence (CL), epi-fluorescent microscopy (FL),scanning electronmicroscopy (SEM), staining, and total organic carbon(TOC) and X-ray diffraction (XRD) analyses.
The aim of this paper is to present a very detailed taphonomic anal-ysis of belemnites focused on fossil–diagenetic features completing theprevious paper about biostratinomic ones (Part I). Biostratinomy andfossil–diagenesis are terms generally employed for designing differenttaphonomic processes affecting the skeletal remains; the conceptfossil–diagenesis is used to comprise all those processes that take placeon the skeletal remains within a sediment or rock (see Part I for refer-ences and more details). In this Part II belemnites of the Upper Jurassic(Oxfordian–Lowermost Kimmeridgian) sediments from the Prebetic
(Betic Cordillera, southern Spain) are analyzed in order to recognize andinterpret the fossil–diagenetic processes including the use of transmittedlight petrography, CL, FL and SEM, in addition to geochemical techniquesinvolving elemental and stable isotope analyses. The combined use ofthese techniques allowed the analysis of belemnite growth and preserva-tion including the timing of the different fossil–diagenetic processes thataffected belemnite rostra. Moreover, this study re-examines the criteriaused for discriminating diagenetically altered and non-altered belemnitesamples in the literature and proposes a set of combined petrographicand geochemical criteria, which should be used for this purpose.
2. Location
Belemnite specimens were collected from the External and InternalPrebetic (Betic Cordillera). During the Late Jurassic, the ExternalPrebetic represented a mid-shelf environment whereas the InternalPrebetic represented the outer shelf to slope environment (Fig. 1).Belemnites come from two sections: Pozo Cañada (from the ExternalPrebetic) and Río Segura (from the Internal Prebetic, Fig. 1). A detailedlocation map is included in Part I.
The Pozo Cañada section (PC) crops out on the northern slope ofthe Sierra del Chortal located in the southeast of the village of PozoCañada (Albacete province). This section is 20 m thick representingthe Middle Oxfordian to lowermost Kimmeridgian (Transversariumto Planula Zone) composed by spongiolithic limestones, spongiolithicmarls–peloidal limestones and marl–limestone rhythmite (Olóriz etal., 2002, 2012).
The Río Segura section (RS) is located in the Sierra de Segura(Albacete province), at the km 20 along the road between the villagesof Yeste and Santiago de la Espada. This section is composed of
Fig. 1. Geological sketch of southeastern Iberia with location of the studied sections: Pozo Cañ
lumpy–oncolitic limestone lithofacies rich in ammonoids representingthe Oxfordian to lowermost Kimmeridgian (Transversarium to PlanulaZone, Olóriz et al., 2002, 2012). The studied section ends with a thickpackage of marls.
3. Methods
The analysis of the fossil–diagenetic features has been focused on 86thin sections (56 from the PC section and 31 from the RS section) select-ed from 289 specimens of belemnites corresponding to Hibolithes, andless commonly to Belemnopsis, although it was not possible to classifymany of the specimens as they were fragmented. One or two polishedand uncovered thin sections for each specimen were prepared to400–500 μm thickness for petrographic studies and geochemical analy-sis. Matched thin sections with a thickness of 30 μm were obtainedfrom six of those specimens for petrographic and microprobe analyses.Thin sections were prepared along longitudinal, preferably, and trans-verse sections of the rostra; and when possible along both sections inthe same specimen. Thin sections were studied and photographedunder transmitted light microscope, cathodoluminescence (CL), andepifluorescent (FL) microscopy: CL entailed the use of a Technosyncold cathodoluminescent unit MK4 at 20–24 kV with 350–400 mAbeam current, whereas incident-light FL was carried out using a NikonY-FL epifluorescence system coupled to an Eclipse 6400 POL petro-graphic microscope. An UV (340–380 nm) excitation filter (400 nm di-chroic mirror and 420 nm barrier filter) and a blue light (450–590 nm)excitation filter (505 nm dichroic mirror and 520 nm barrier filter)were used for petrographic work.
The analysis of belemnites in thin sections sheds light on multipletaphonomic processes, including the biostratinomic (see Part I) and
ada (PC) and Rio Segura (RS) sections. See Belemnite taphonomy: Part I for more details.
fossil–diagenetic alteration, while facilitating precise geochemicalanalyses.
Geochemical analyses were performed separately in well-preserved(non-altered) and/or in petrographically altered portions of 86 belem-nite rostra. Altered areas defined by the petrographic study weremapped, both on the photographs and on the thin sections, using apermanent color marker. Subsequently, non-altered portions of thespecimens and petrographically altered areas were microsampledseparately and directly from the thin sections in order to comparegeochemical and isotopic results. A microscope-mounted drilling sys-temwas used for sampling directly from thin sections, so that carbonatematerial was obtained only from selected areas. Sampling was accom-plished using 0.02 to 0.5 mm dental burrs, resulting in approximately500 μg of carbonate powder for both elemental and C and O isotopeanalyses. This method of sampling allowed full control of the sampledobject, avoiding any microfractures or microborings within the speci-mens. When possible, different analyses of the same specimen anddifferent specimens from the same stratigraphic level were analyzedin order to record the intra-specimen and inter-specimen geochemicalvariability.
ICP-MS elemental analysis (Ca, Mg, Sr, Mn and Fe), and C and O sta-ble isotopes, were performed at the University of Michigan. In bothcases, sample powders were roasted in vacuo for one hour at 200 °Cto remove volatile organic contaminants. For elemental ICP analysis,carbonate powder was dissolved in a 2 ml solution consisting of 2%HCl and 1% HNO3 with 1 ppb of an internal standard. Cation concentra-tions of Sr, Mg, Fe, Mn and Ca were determined simultaneously byICP-MS (Finnigan Element). The analytical precision for all minorelements was approximately ±5%. For δ13C and δ18O analyses, andafter roasting, the samples were reacted at 73 °C in an automatedcarbonate reaction system (Kiel-IV) coupled directly to the inlet of aFinnigan MAT 253 gas ratio mass spectrometer. Isotopic ratios werecorrected for 17O contribution and are reported in per mil notationrelative to the VPDB standard. Values were calibrated using NBS 19 asthe primary standard, while analytical precision was monitored bydaily analysis of NBS powdered carbonate standards. The measuredprecision was set at better than 0.1‰ for δ13C and δ18O.
Backscattered scanning electron microscopy (BSEM) and electronmicroprobe analyses were performed using a JEOL JXA-8900 electronmicroprobe equipped with five wavelength-dispersive spectrometersat the Universidad Complutense de Madrid. Spot analyses and/orseveral continuous elemental profiles for Mg, S, Sr, Fe, and Mn wereperformed for three well-preserved belemnite specimens (PC-31-41,PC-37-04a and PC-45-14b). An accelerating voltage of 15 kV, beamcurrent of 10 nA and a spot size of 10 μm were used for spot analyses;continuous profiles entailed an accelerating voltage of 20 kV, beamcurrent of 100 nA, and spot size and step interval of 1 μm diameter(dwell time=1000 ms). The counting time for spot analyses was 30 sper element; given that 5 elements could be analyzed simultaneously,the total time for each analysis (Ca, Mg, Sr, Fe, Mn and S) was 60 s.The relatively low voltage, beam current and counting time employedfor spot analyses minimized the damage of calcite under the electronbeam although increased the detection limits for some elements, suchas the Fe and Mn. Nevertheless, Fe and Mn contents obtained frommicroprobe have not been used for interpreting altered or non-alteredareas. Measured precision was better than ±0.12% for Mg, ±0.24% forS, ±0.35% for Sr, ±1.6% for Fe, and ±1.5% for Mn, and the detectionlimits ranged between 170 and 200 ppm for Mg, 200 and 250 ppm forSr, 220 and 300 ppm for S, 410 and 600 ppm for Mn and 370 and600 ppm for Fe.
4. Results
The conditions of preservation of belemnite rostra are controlledin part by their original low-magnesium calcite composition, whichmakes them relatively stable and resistant to diagenetic alteration
(Veizer, 1974; Sælen, 1989; Podlaha et al., 1998; Rosales et al., 2001,and references therein). Notwithstanding, it is common for somebelemnites, or parts thereof, to be diagenetically altered, as evidencedby petrographic and/or geochemical data (e.g. Spaeth et al., 1971;Veizer, 1974; Sælen, 1989; Podlaha et al., 1998; Rosales et al., 2001).
4.1. Petrographic evidence of fossil–diagenetic alteration
A number of authors have shown that non-diagenetically-alteredbelemnites display a transparent appearance under transmitted lightand anabsence of luminescence,while diagenetically-altered belemnitestypically have a cloudy appearance under transmitted light and a brightto dull yellowish, orange or brownish luminescence (e.g. Sælen, 1989;Rosales et al., 2001 and references therein).
The belemnites in the PC and RS sections commonly show thesetraits, though the degree and extension of fossil–diagenetic alterationcan vary among specimens and/or among areas of a single specimen(Fig. 2). Diagenetic alteration was seen to be similar in both the PCand RS sections. One noteworthy feature, apparently distinctive ofsamples from the PC section, is a higher degree of diagenetic alter-ation (in particular, those obtained in spongiolithic and marly facies),as compared to a higher degree of fracturing in samples obtainedfrom the RS section. Belemnites obtained from the RS section further-more have typically a translucid and brownish appearance (Fig. 2)and emanate a fetid smell when drilled for geochemical purposes.
In petrographical terms, some of the studied belemnite rostra arewell-preserved (Figs. 2A1, B, 3A, B), with a transparent to translucentappearance under transmitted light (TL), an absence or near-absenceof inclusions, and a lack of luminescence (non-luminescent, NL), de-spite the fact that the most external growth rings and apical linemight display a cloudy to opaque appearance, an abundance of inclu-sions, and bright to dull luminescence.
Most specimens, however, are moderately well-preserved (Fig. 2A2).These specimens commonly display cloudy and luminescent apicalregions and an alternation between transparent to translucent andNL concentric growth areas (“laminae pellucidae” sensu Müller-Stoll,1936) and concentric growth areas, generally up to 1 mm thick, inwhich cloudy and luminescent growth rings predominate (“laminaeobscurae” sensu Müller-Stoll, 1936) (Figs. 3C–E, 4). The epifluorescence(FL) study performed showed that cloudy and luminescent growthrings and apical lines displayed more intense FL than NL portions of therostra (Fig. 4A–G). At a closer view, NL and transparent growth areasare seen to be constituted by concentric layers (10–50 μm thick general-ly) displaying a radial structure and separated by very thin inter-radiallayers (generally less than 5 μm thick) that do not show the radial struc-ture (Fig. 4D). This alternation between layers displaying radial structureand very thin interradial layers resembles those described by Sælen(1989) in belemnite rostra after etchingwith glutardialdehyde, revealingorganic-matter-binding properties. The transparent and NL growth areasdisplaying radial structure show non-, weak or intense FL, which is oftenfinely zoned (Fig. 4D–L). The very thin layers separating those with theradial structure typically display intense and homogeneous FL, evenwhen they have a clear or transparent appearance and no luminescence(Fig. 4D–F); in some cases these thin layers display a cloudy or opaqueappearance, bright to dull luminescence, and intense and patchy FL(Fig. 4D–F).
The FL pattern is observed in most of the well-preserved andmoderately well-preserved rostra. This small-scale zoning observedunder FL of NL growth areas can also be observed under BSEM, thoughin many cases it is less evident than with FL (Fig. 4H–L). When evident,the slightly fluorescent rings appear light gray in color under BSEM,while the more intensely fluorescent rings are seen to have a darkergray tone under BSEM (Fig. 4H–L).
The boundaries between the transparent NL growth rings and thecloudy luminescent ones may be straight, yet tend to be irregular(Fig. 3C–E). CL petrographic study revealed that cloudy growth rings
Fig. 2. Photographs showing thin sections of belemnite rostra, which has been used for petrographic and geochemical studies. Red arrows indicate the diagenetically altered areas. Yellowarrows indicate fractures. White arrows indicate stylolites. A. Transverse sections of belemnite rostra. Note the radial fractures affecting belemnite rostra (yellow arrows): A1: Very wellpreserved specimen (PC-45-14a), A2: Moderately preserved specimen (PC-37-4a), A3: Poorly preserved specimen (PC-11-41), A4: Very poorly preserved (PC-6-23c). B. Longitudinalsections of well preserved belemnite rostra: B1: Specimen from the PC section (PC-53-25), B2: Specimen from the RS section (RS-12-18). In well-preserved specimens only the apicalregion and/or some growth rings are diagenetically altered and display a cloudy and opaque appearance. Non-altered portions of the rostra display transparent to translucent appearance.C. Longitudinal sections of moderately well-preserved specimens. C1: Specimen from the PC section (PC-47-4). C2: Specimen from the RS section (RS-37s-56). In moderatelywell-preserved specimens some portions of the belemnite rostra are diagenetically altered and display a cloudy and opaque appearance. Non-altered portions of the rostra display trans-parent to translucent (NA) appearance. D. Longitudinal sections of poorly preserved specimens. D1. Specimen from thePC section (PC-13-11). D2: Specimen from theRS section (RS-4-65).In poorly preserved specimens altered portions of the rostra, displaying cloudy appearance, predominate over non-altered areas, or even the whole belemnite rostra may be altered.E. Transverse fractures affecting belemnite rostra. E1: Specimen from the PC section (PC-11-183). Note that fractures cross-cut both altered and non-altered portions of the belemnitesand in some case fractures act as boundaries for alteration (yellow arrow). E2. Specimen from the RS section (37i-10). Note the translucent caramel color of non altered portion of therostra compared to the fractures. F. Stylolites affecting belemnite specimens (white arrows). F1: Specimen from the PC section (PC-59-60). F2: Specimen from the RS section(RS-5-10). Scale bars=1 mm.
underwent partial or total dissolution; this finding is supported bythe irregular shape of the boundaries between many of these rings,suggesting a selective dissolution postdated by precipitation of calcite
cement, which may produce a luminescent zoning pattern indicativeof crystal growth within a pore (Fig. 3D). It has also been observedthat diagenetic alteration expands from the cloudy and luminescent
Fig. 3. A. Microphotographs of a well-preserved belemnite rostrum (right: transmitted light, TL; left: cathodoluminescence, CL). Note the transparent appearance under TL and theabsence of luminescence of the rostrum. The only altered areas correspond to a longitudinal fracture developed in the apical region and the outermost rings that displays brightorange luminescence. B. CL microphotograph showing a well preserved specimen, in which only the apical region is diagenetically altered displaying bright orange-yellowishluminescence. Non-altered portions of the rostrum are non-luminescent. C. Microphotographs of a moderately well-preserved belemnite rostrum (right: TL; left: CL). Note thetransparent appearance under TL and the absence of luminescence of non-altered areas of the rostrum. Altered areas display cloudy appearance and bright orange luminescenceand correspond to some growth rings (white arrows) and some radial structures, which resemble the boundary between calcite crystals (yellow arrow). D. CL microphotographshowing alternation of altered (luminescent) and non-altered (non-luminescent, NL) concentric growth rings. The limits between NL growth rings and luminescent ones can bestraight (green arrow) or irregular (blue arrows). Note that there are irregular areas along some growth rings (yellow arrows), which underwent dissolution postdated by calcitecement precipitation. Calcite cement displays a thin yellowish zoning first and subsequently brownish dull luminescence. Note the radial and luminescent thin areas (whitearrows), which correspond to the boundaries between calcite crystals. E. Microphotographs of a poorly preserved belemnite rostrum (right: TL; left: CL), showing the alternationbetween the non-altered transparent and NL growth rings and the altered cloudy and bright to dull orange luminescent growth rings. The boundary between altered andnon-altered areas can be straight (green arrows) or irregular (blue arrows). Where diagenetic alteration is maximum belemnite rostrum displays a cloudy appearance and unzonedbright to dull luminescence (yellow arrow). F. CL microphotograph showing a belemnite rostrum displaying an inner and well-preserved NL area, surrounded by two very poorlypreserved and largely diagenetically altered areas displaying unzonning bright orange luminescence. Scale bars=500 μm.
areas favored by the boundaries between single rostra crystals(Fig. 3D) or by fractures (Fig. 5A), which may act as a boundarybetween non-altered and altered areas of the rostrum (Fig. 5B). Theareas of the rostra with abundant microborings might display, insome specimens, a higher degree of diagenetic alteration. Likewise,some samples show that microborings cross-cut the external growthzoning, as observed under TL, CL and FL (Fig. 6).
Finally, certain specimens or portions of some specimens (mainly theapical line and the rostrum cavum) are poorly or very poorly-preservedpetrographically, displaying alternating dark dull to light dull or brightluminescence, or homogeneous bright to dull luminescence (Figs. 2A3,4,C, D, 3E, F). In these samples, mostly obtained from the spongiolithicand marly facies of the PC section, diagenetic alteration is seen to extendfrom the apical line, rostrum cavum, and/or the growth rings to the
adjacent areas of the rostrum solidum, or even to the whole belemniterostrum. The worst-preserved areas have no growth zoning, a homoge-neous cloudy appearance under TL, and a homogeneous bright to dull lu-minescence and intense FL (Fig. 3F).
4.2. Fracturing
Belemnite specimens are affected by two different types of fractures,not necessarily producing isolated fragments. First, fine longitudinalfractures along the axis of the belemnites display a radial pattern whena transverse cross-section of the rostra is examined (Figs. 2A, 7A, B).These fractures, which may show a braided structure in the longitudinalsection, have bright luminescence and intense FL (Fig. 2A); and secondly,transverse to oblique fractures, irregular and generally wider than thelongitudinal ones. Transverse fractures are more abundant in specimensfrom the RS section, being commonly filled by sparry calcite cementthat may display zones of orange–brownish bright to dull luminescence,with weak or absent FL (Figs. 2E, 4A–C, 5A, B). Fractures affecting belem-nite rostra cross-cut diagenetically altered growth rings and apical lines(Figs. 2E1, 7C, D). However, depending on the specimen, transverse frac-tures have a different chronology relative to the longitudinal ones.
4.3. Stylolites
Though less common than fractures, stylolites are also observed(Figs. 2F, 5C, D), and generally parallel to the longitudinal axis, as zigzagdark lines under transmitted light microscopy and showing bright todull luminescence (Fig. 5C, D). Occasionally, they are also observedperpendicular to the longitudinal axis (Fig. 7F). Stylolites cross-cut thediagenetically altered growth rings and the apical line. The parageneticrelationship between stylolites and fractures is not always clear, althoughin some specimens stylolites cross-cut transverse fractures.
4.4. Geochemical evidences of fossil–diagenetic alteration
4.4.1. Trace elementsSeveral authors have established that, as a general rule, diagenetical-
ly altered belemnites contain higher Fe and Mn contents and lowerSr/Mn ratios than well-preserved belemnites (e.g. Veizer, 1974; Veizerand Fritz, 1976; Jones et al., 1994a, 1994b; Price and Sellwood, 1997;Rosales et al., 2001; Wierzbowski, 2004; Wierzbowski and Joachimski,2007), suggesting that samples with low Mn (b30–50 ppm), low Fe
Fig. 4. A. Photomicrograph showing a longitudinal section of a moderately well-preserved band some concentric growth rings display a cloudy appearance. The rest of the rostra displayrostra. The red square on the left represent the area photographed in D. B. Same image as Aorange luminescence. Calcite cement filling the transverse fracture (yellow arrow) display zophotographed in E. C. Same image as A and B under blue-light epi-fluorescence microscopycence. Non-altered, NL areas display FL zoning. Calcite cement filling the transverse fracturephotographed in F. D. Detailed TL photograph of A. Note that transparent growth areas aseparated by very thin inter-radial layers, which do not show the radial structure (white ainter-radial layers, which are commonly diagenetically altered displaying cloudy appearance(both with and without the radial structure, red and white arrows, respectively) are NL. Theluminescence (blue arrows). F. Same image as D and E under BFL. Transparent and NL growzoned (red arrow). The thin layers separating the ones with radial structure display intendiagenetically altered (blue arrows) display very intense yellowish to greenish but patchyspecimen (PC-45-14b) under TL (central area), CL (lower area) and BFL (upper area). Theexternal part of the rostra, which display cloudy appearance under TL, bright orange lumineand display zoning fromweak to intense FL. Yellow arrowpoints to the area of the rostrawhereshowing a transverse section of a moderately well-preserved belemnite specimen (PC-37-4a) uare transparent. Yellow arrow points to the area of the rostra where a continuous profile wphotographed in J. I. Same image as H under BFL. Diagenetically altered areas display intenserepresents the area photographed in L. J. Detailed TL photograph of H. Note that a concentrarrow points to area of the rostra where a continuous profile was performed with the microgray and lighter gray, which are equivalent to that observed under TL and FL (red transverse lFL. The long red line corresponds to the area where the continuous profile was performed. Nounder BSEM and are more porous than non-altered transparent areas. L. Same image as J anBSEM. Note that the weak slightly fluorescent rings show light gray color under BSEM andbars=500 μm. Black scale bars=200 μm.
(b150–250 ppm), high Sr content (>950–1150 ppm) and/or Sr/Mnratios >80 are representative of the original marine geochemistry.
In this study, major and trace elements of petrographically alteredand/or non-altered areas of 86 belemnite rostra were analyzed usingICP-MS (Table 1; Fig. 8). It was found that the elemental geochemistryof altered and non-altered areas is distinctive; as established by theabove authors; Fe and Mn contents are generally higher in alteredsamples, a finding that is particularly evident in the 16 specimensfrom which non-altered and altered portions of the same belemnitespecimen were analyzed (Fig. 8 and Table 1).
Petrographically non-altered samples have Mn contents b15 ppm,with a mean value (mv) of 5 ppm. Just one sample (number 61,Table 1, Fig. 8), which was apparently well-preserved petrographically,contained 78 ppm of Mn. Fe content of the non-altered samples yieldedvalues between 4 and 299 ppm (mv=44 ppm of Fe), though threesamples had an Fe content over 125 ppm (Fig. 8 and Table 1); in generalsamples from the PC section had slightly lower Fe content thanthose from RS (mv=37 and 57 ppm, respectively). Mg/Ca ratiosof non-altered samples ranged between 7.2 and 28.1 mmol/mol (mv=14.8 mmol/mol), Sr content between 889 and 1307 ppm (mv=1100 ppm of Sr), and Sr/Mn ratios were between 83 and 688 (mv=332). However, sample number 61, containing 78 ppm of Mn, had aSr/Mn ratio of 14 (Table 1, Fig. 8).
Petrographically altered samples showed much higher Mn and Fecontents than the non-altered samples, with mean Mn and Fecontents of 106 ppm and 838 ppm, respectively (Fig. 8 and Table 1).The Sr content and Sr/Mn ratios of altered samples respectivelyranged between 185 and 1373 ppm (mv=773 ppm of Sr), and be-tween 0.3 and 338 (mv=60), both values lower than those obtainedfrom non-altered samples. Mg/Ca ratios of altered samples werebetween 8 and 30.9 mmol/mol (mv=15 mmol/mol), in this casesimilar to those obtained from well-preserved samples (Fig. 8 andTable 1). Nevertheless, it is important to note that some of thesepetrographically altered samples have Fe, Mn content and Sr/Mnratios within or very close to the range of values given as representativeof the original marine geochemistry by the authors mentioned above(for example, samples 5, 15, 18, 57, 58, 62, 75, 85 and 86; Fig. 8 andTable 1).
BSEM images of the rostra performed with the electron microprobeshow that non-altered transparent and NL growth rings display a weak,yet observable fine alternation of light and dark gray rings resemblingthe pattern observed under FL (Fig. 3J–L). The continuous profiles
elemnite specimen (PC-31-4) under TL. Diagenetically altered areas as the apical regiona transparent appearance. There is transverse fracture (yellow arrow) cross-cutting theunder CL. Non-altered areas are NL and the diagenetically altered areas display brightned brownish dull to non- luminescence. The red square on the left represents the area(BFL). Diagenetically altered areas display more intense greenish to yellowish fluores-(yellow arrow) displays non- to weak FL. The red square on the left represents the areare constituted by concentric layers displaying radial structure (red arrow), which arerrow). Layers displaying radial structure commonly are better preserved than the thin(blue arrows). E. Same image as D under CL. Non-altered transparent concentric layersthin inter-radial layers, which are cloudy (diagenetically altered) display bright orangeth areas displaying radial structure show non-, weak or intense FL, which is often finelyse yellowish to greenish and homogeneous FL (white arrow). Thin layers, which areFL. G. Photomicrograph showing a transverse section of a well-preserved belemniteonly diagenetically altered areas correspond to a few concentric growth rings of the
scence and intense yellowish–greenish patchy FL. Non-altered areas are transparent, NLa continuous profilewas performedwith themicroprobe (see Fig. 9B). H. Photomicrographnder TL. Diagenetically altered growth rings show a cloudy appearance. Non-altered areasas performed with the microprobe (see Fig. 9A). The blue square represents the areagreenish to yellowish fluorescence. Non-altered areas display FL zoning. The blue squareic zoning is observed even within the non-altered transparent areas (red lines). Yellowprobe (see Fig. 9A). K. Same image as J under BSEM. Note the concentric zoning of darkines, see Fig. 4J and L), although in many cases is not as evident and that observed underte that diagenetically altered cloudy areas under TL commonly display a light gray colord K under BFL. The FL zoning pattern is also equivalent to that observed under TL andthe more intense fluorescent rings show a darker gray tone under BSEM. White scale
performed for three specimenswith themicroprobe forMg, Sr, S, Fe andMn along thesefinely zoned non-altered areas reveal cyclical changes inthe Mg and S content; that is, growth rings displaying weak or non-FLand light gray color under BSEM have lower Mg and S content thangrowth rings displaying more intense FL and darker gray color underBSEM. No significant changes were noted for Fe, Mn or Sr contentover the profiles (Fig. 9), though higher Fe and Mn contents werereflected by altered cloudy and intensely fluorescent areas. Spot analy-ses performed along the profiles (Fig. 10A, Table 2) also show variationinMg and S concentrations, even though the absolute values of both ele-ments are different in the two specimens analyzed (Fig. 10A). Specifically,specimen number 32 (PC-31-41) yields Mg values between 1061 and4004 ppm (mv=2305 ppm, n=92) and S content between 693 and2743 ppm (mv=1358, n=92); yet specimen number 34 (PC-37-4a)
yields Mg values between 2424 and 5132 ppm (mv=3642 ppm, n=27) and S content of 661 to 2103 ppm (mv=1171, n=27). We shouldstress that the absolute Mg and Sr concentrations obtained with the mi-croprobe in both specimens analyzed are higher than those obtainedwith ICP (Tables 1, 2), a finding that may be attributed to the lower pre-cision of themicroprobe as comparedwith the electronmicroprobe, and/or because the areas analyzed with ICP and the electron microprobe inthe two specimens were different.
4.4.2. Stable isotopesStable isotope data obtained from belemnites samples are shown in
Table 1 and Fig. 8. Petrographically non-altered samples (n=77) yieldδ18O values between +0.19 and −0.96‰ (mv=−0.37‰) and δ13Cvalues between +1.24 and −1.21‰ (mv=+0.45‰); in contrast,
Fig. 5. A. CL photograph showing a transverse facture cross-cutting a belemnite rostrum. A transverse fracture (white arrow) postdates diagenetic alteration of the apical region andgrowth rings. Calcite cement filling transverse fracture displays zoned non- to dull luminescence, which is, in turn, postdated by a thin longitudinal fracture developed in the apical region(white arrow). Note that diagenetic alteration of the rostrum is enhanced where several fractures are developed (yellow arrow). B. CL photograph showing a transversal facturecross-cutting a belemnite rostrum. Calcite cement filling the fracture displays dull brownish luminescence. Note the fracture act as a boundary between areas of the rostra with differentdiagenetic alteration (yellow arrow). C. CL photograph of a transverse section of a belemnite rostrum displaying alternation between non- and bright orange luminescence growth rings.Stylolite (blue arrow) developed subsequently and affects growth rings. They have a serrated (zig-zag) pattern and display dull brownish to orange luminescence. D. CL photograph of alongitudinal section of a belemnite rostrum. As in C, stylolites (blue arrow) have serrated pattern and display bright orange luminescence. In this case they are affecting the apical region.Scale bars=500 μm.
petrographically altered samples (n=28) yield δ18O values between+0.12 and −6.22‰ (mv=−1.76‰) and δ13C values between +1.22and −1.92‰ (mv=+0.3‰).
Some criteria based on stable isotopes were established to test theeffects of diagenesis on the original marine isotope signatures ofancient carbonates (e.g. Lohmann, 1988; Rush and Chafetz, 1990;Marshall, 1992; Jenkyns et al., 1994; Sælen et al., 1996; Veizer et al.,1997; Bruckschen et al., 1999; Rosales et al., 2001; Nieto et al., 2008 andreferences therein). For example, it has been argued (e.g. Lohmann,1988; James and Choquette, 1990; Marshall, 1992; Rosales et al., 2001)that with diagenetic alteration the δ18O and δ13C values of carbonates,as well as belemnite calcite, tend to be more negative and to covary.Rosales et al. (2001) further point out that the lack of correlation ofSr, Fe and Mn contents with δ18O of non-luminescent and slightly lumi-nescent belemnites suggests minimal postdepositional alteration. Ourdata agree only in part with these statements. For instance, δ18O valuesof petrographically altered portions of the belemnites are more negative(between 0.04 and 5.4‰) than non-altered portions of the same speci-mens (Figs. 8, 10B). Yet δ13C values do not show the same trend, assome altered portions of the belemnites have more negative δ13C valuesthan the non-altered ones (0.07 to 2.7‰ more negative), while otherspecimens show the opposite trend— the altered areas have more posi-tive δ13C values than non-altered ones (0.16 to 0.8‰ more positive;Figs. 8, 10B). As for the covariance between different parameters,non-altered samples show no any correlation between δ18O values andδ13C values, or between δ18O values and Sr, Fe, Mn, or Mg contents;altered samples show some correlation between δ18O values and δ13C
values, but none between δ18O values and Sr, Fe, Mn or Mg contents(Fig. 10C–F).
5. Interpretation
5.1. Petrographic evidence of belemnite alteration
The petrographic study indicates that diagenetic alteration of bel-emnite rostra started before burial, as a biostratinomic process, andcontinued during progressive burial, as a fossil–diagenetic process.
As interpreted before by a number of authors (e.g.Müller-Stoll, 1936;Sælen, 1989; Podlaha et al., 1998; Rosales et al., 2001;Wierzbowski andJoachimski, 2007) transparent and NL areas of the belemnite rostra(equivalent to the “laminae pellucidae” of Müller-Stoll, 1936) most like-ly retain their original structure andmineralogy and are hardly diagenet-ically altered. Moreover, Sælen (1989) interpreted the concentric ringsas true growth rings, pointing out that “the rostrum is made up of com-posite radial structures which accreted periodically to give a pattern ofconcentric growth-rings”, recognizing that the pattern under blue lightFL (BFL) asmore or less fluorescent growth rings corresponds to growthincrements with varying amounts of organic matter, intra- andintercrystalline, distributed throughout the rostrum. Wierzbowski andJoachimski (2009) observed a pattern similar to that described bySælen (1989) in belemnite rostra etched with glutardialdehyde, whichthey interpret as indicative of different densities and contents of organicmatter, and thus as true growth increments of the rostra. Our data are inagreement with these authors; transparent and NL growth areas are
Fig. 6. A. TL microphotograph of the external part of a belemnite rostrum, which is largely bored (Type B single and more or less sinuous tube-like microborings, see Part I for moredetails). Borings, which are filled with micrite, cross-cut (red arrows) and, thus, postdate the concentric transparent and cloudy growth layers of the belemnite rostra. B. Sameimage as A under CL. Micrite filling borings display bright orange luminescence. Non-altered transparent growth layers are NL and diagenetically-altered cloudy growth ringsdisplay brownish luminescence. Note that borings affect cloudy and luminescent growth rings (red arrows). C. Same image as A and B under ultraviolet fluorescence (UVFL). Micritedisplays bluish luminescence. Non-altered transparent and NL growth areas display a FL pattern from non-, weak to more intense brownish FL. Diagenetically altered and cloudyareas display a patchy FL. Note that borings crosscut the concentric growth pattern, including cloudy layers, of the belemnite (red arrows). D. TL microphotograph of the externalarea of a belemnite rostrum, which is largely bored. Borings are more abundant towards the external-most part of the belemnite (the upper part of the photograph). Observe theconcentric cloudy and transparent growth pattern (horizontal in the photograph) and note that some microborings (Type B tube-like microborings) are more abundant and evenstart to develop in the cloudy areas (red arrows). E–F. TL (left) and UVFL (right) microphotographs of the external part of a belemnite rostra, which is largely bored (Type C sac-likemicroborings, 80–300 μm in diameter, connected with the surface of the rostrum by a short, narrow tube, see Part I for more details). Borings, which are filled with bluish FL micrite,cross-cut the concentric transparent and cloudy growth layers of the belemnite rostra. Note that some Type C sac-like microborings are more abundant and even start to develop inthe cloudy areas (red arrows). Note also the concentric FL growth pattern within the non-altered transparent areas of the rostra. Scale bars=250 μm.
formed by an alternation of concentric layers displaying radial structureand FL zoning, and very thin calcite inter-radial layers with intense FL(Fig. 4D–F). When analyzed with themicroprobe, the FL concentric pat-tern is seen to be clearly related to Mg and S contents; accordingly, themore intense the FL, the higher the Mg and S content of calcite, yetthere are no significant changes in the Mn, Fe or Sr content (Fig. 9).There is evidence that photoluminescence (fluorescence and phospho-rescence excited at optical wavelengths) in calcite can be activated by or-ganic compounds (e.g. VanGijzel, 1967; Boto and Isdale, 1985; Cercone etal., 1985; Pedone et al., 1990; Burrus, 1991) and trace elements, such as
Mn2+ (e.g. Aguilar and Osendi, 1982; Walker et al., 1989; Pedone et al.,1990). In our case, calcite that displays this FL pattern does not showany CL (Fig. 4A–G), which is typically activated in sedimentary calcitesby Mn2+ (e.g. Machel and Burton, 1991; Machel et al., 1991). Further-more, both ICP and microprobe analyses show that NL calcite has no orvery little Mn2+ (less than 15 ppm, Fig. 8, Table 1). In addition, nochanges in Mn content were observed between very intense and veryweak FL (Fig. 9),making it improbable that the concentric pattern offluo-rescence was caused by changes in Mn content. On the contrary, asshown in Fig. 9, the intense FL concentric layers have higher Mg and S
Fig. 7. Fracturing. A. Specimen (PC-47-19, marl–limestone rhythmite, Planula Zone) showing transverse and longitudinal fractures (following the apical line). B. Transversefractures limiting different zones of diagenetic alteration (PC-12-13, spongiolithic limestone, Transversarium Zone). C. Transverse fracture with secondary infilling (RS-36-136,lumpy limestone, Planula Zone). D. Fracture limiting the diagenetic alteration and showing calcitic infilling. Note the microsparite growth in the internal most chambers of thephragmocone (PC-11-183, spongiolithic limestone, Transversarium Zone). E. Braided fractures affecting the totality of a specimen (PC-48-35, marl–limestone rhythmite, PlanulaZone). F. Classic example of stylolite (PC-43/44-21, spongiolithic marl-peloidal limestone, Bimammatum Zone). Scale bars=1 mm.
concentrations than those showingweak or no FL. Thus, the concentric FLpattern was most likely activated by organic compounds incorporatedinto the belemnite rostra during progressive growth and not as a diage-netic process, along with the associated changes in theMg and S concen-tration observed over the profiles. In this sense, McArthur et al. (2007a)andWierzbowski and Joachimski (2009) report changes inMgand S con-centrations similar to those obtained in this work (Fig. 10A, Table 2) andthose reported for modern marine biogenic LMC (Busenberg andPlummer, 1985). Sulfur is present in a variety of biogenic and diageneticcarbonates as sulfate substituting for carbonate (Pingitore et al., 1995);for example, according to Wierzbowski and Joachimski (2009), sulfurmay be present as SO4
−2 in the calcite lattice, co-precipitatedwith belem-nite calcite or incorporated during earlymarine diagenesis, and as organ-ically bound S−2, which may have been incorporated and bound withorganic matter present in the rostrum. This is in line with our
observations, since the highest concentrations of S are observed wherethe FL (presumably activated by organic compounds) is most intense(Fig. 9). The changes in the Mg within a single belemnite rostra havebeen interpreted as showing the effects of temperature change duringgrowth, overprinted by the effects of biofractionation within the belem-nite rostrum (McArthur et al., 2007a) and as an ontogeneticbiofractionation effect (Wierzbowski and Joachimski, 2009).
Regarding the cloudy and luminescent growth rings (the “laminaeobscurae” of Müller-Stoll, 1936), our findings suggest that they havebeen diagenetically altered, as interpreted by previous authors (e.g.Müller-Stoll, 1936; Sælen, 1989; Podlaha et al., 1998). CL petrographicstudy reveals that cloudy growth rings underwent selective dissolutionfollowed by precipitation of calcite cement (Fig. 3D). This dissolution–precipitation process typically affects the thin and fluorescentinter-radial layers (Figs. 3C, D, 4D–F), leading them to acquire a cloudy
appearance and bright to dull luminescence and intense FL, and it mayextend partially to the adjacent layers, displaying the radial structures(Fig. 3D, E). Calcite luminescence, as mentioned above, is attributedto the incorporation of Mn2+ in the calcite lattice, introduced undersuboxic and/or anoxic precipitation conditions (e.g. Machel andBurton, 1991; Machel et al., 1991). On the other hand, Fe2+, which en-ters the calcite lattice under reduced condition, is a quencher of the lu-minescence (e.g. Machel and Burton, 1991; Machel et al., 1991),meaning that changes in the amount of Mn2+ and Fe2+ incorporatedinto the calcite during progressive precipitation would lead to the CLzoning observed in luminescent cloudy rings (Fig. 3D). Nevertheless, aprocess of recrystallization of some of the cloudy and luminescentrings cannot be discarded, particularly in those showing no zoning inthe calcite and having homogeneous luminescence (Fig. 3D–F).As pointed out above, FL in calcite may result from the incorporationof Mn and/or organic matter. Therefore, the intense FL observedin these cloudy layers could be caused by the incorporation of Mn.Still, organic matter cannot be ruled out as an activator of the FL of thecloudy layers; indeed, Müller-Stoll (1936) interpreted the “laminaeobscurae” as organic layers in origin, while Sælen (1989), based onthe etching of belemnite rostra with weak acids, hydrogen peroxideand glutardialdehyde, inferred that the non-radial layers representedorganic-rich areas.
Another matter is when the dissolution of cloudy growth rings andthe apical line of well and moderately well-preserved belemnite rostraactually occurred. In some samples microborings are seen to cross-cutthe external growth zonation (Fig. 6), moreover, in some samplesType B microborings (single and more or less sinuous tube-likemicroborings, ranging from 13 to 25 μm in diameter, see Reolid andBenito, 2012-this issue for more details) developed preferably withinthe cloudy growth rings (Fig. 6D–F), suggesting that the porosity and/or microenvironment of these rings were more suitable for the micro-organisms to bore, perhaps because they were richer in organic matter.Thus, diagenetic alteration of the cloudy growth rings probably oc-curred as a biostratinomic process very shortly after the death of thebelemnites and before burial. Similarly, Tan and Hudson (1974), Bayer(1975) and Sælen (1989) point out that the degradation of organic ma-terial in belemnite rostrawould have started early in their post-mortemhistory and might have increased the original porosity or createdporous zones. The decay of organic matter could also have favored thereduced environment suitable for the incorporation of Mn2+ and/orFe2+ into the cloudy calcite lattice.
Poorly and very poorly-preserved specimens typically display acloudy appearance under TL, zoned or homogeneous bright to dullluminescence, and intense zoned or homogeneous FL, with no evidenceof extensive dissolution (Fig. 3F). In some samples (Figs. 2C–E and 3E)diagenetic alteration extends from the more easily altered areas of therostrum (the apical line, the apex, the rostrum cavum and/or the cloudygrowth rings) to the adjacent areas of the rostrum solidum or evento the whole belemnite (Figs. 2C–E and 3E). It is probable that thisinvolved a recrystallization process (that is, small scale dissolutionand precipitation) as reported by other authors in belemnite rostra(e.g. Al-Aasm and Veizer, 1986; Sælen, 1989). Such a processwould be enhanced by the presence of fractures and borings and bythe boundary between calcite crystals of belemnite rostra (Fig. 5A, B),favoring the action of diagenetic fluids during progressive burial of thebelemnites.
Longitudinal and transverse fractures cross-cut borings, concen-tric growth zoning, the apical line and recrystallized areas, meaningthey developed afterwards during progressive burial. Longitudinalfractures are commonly thin and display bright luminescence andFL. Transverse fractures are generally thicker and are filled by sparrycalcite cement, with un-zoned or zoned orange–brownish dull lumi-nescence and an absence of FL (Figs. 4A–C, 5A, B), suggesting thatcalcite precipitated under reducing conditions and during mechanicalcompaction.
Stylolites developed as a consequence of a pressure-dissolution pro-cess during progressive burial. The paragenetic relationship betweenstylolites and transverse fractures is not clear; the two processes mayhave occurred simultaneously or very close in time due to lithostaticpressure and/or tectonics.
5.2. Geochemical evidence of diagenetic alteration
5.2.1. Trace elementsPetrographically well-preserved belemnite samples have Mn and Fe
contents lower than 15 ppm and 125 ppm, respectively, and Sr/Mnratios higher than 80 (Fig. 8, Table 1). Again, these values are in linewith those reported previously in well-preserved belemnites, and maybe representative of the original marine geochemistry (e.g. Veizer,1974; Jones et al., 1994a, 1994b; Price and Sellwood, 1997; Rosales etal., 2001; Wierzbowski and Joachimski, 2007). Meanwhile, the averageMn and Fe contents obtained from petrographically non-altered samples(mv=5 and 44 ppmofMn and Fe, respectively, Table 1) aremuch lowerthan those given by the above authors as indicative of original marinegeochemistry (>30–50 and >150–250 ppm ofMn and Fe, respectively),being closer to the values obtained by Wierzbowski and Joachimski(2009) from well-preserved belemnites (>5 and >25 ppm of Mn andFe, respectively). Just three samples contain higher Fe and/orMn content(sample numbers 46, 61 and 71). Samples 46 and 71 have, respectively,299 and 190 ppm of Fe; yet both have Mn, Sr and Sr/Mn ratios inthe range of the original marine values inferred by the aforementionedauthors (Table 1, Fig. 8), and the δ18O values of both samples lie withinthe range of δ18O values obtained from well-preserved specimens. It isprobable, then, that the higher Fe content of these two samples is dueto contamination by pyrite or Fe oxides/hydroxides. The petrographicallynon-altered portion of sample number 61 is high in Fe and Mn content(203 and 78 ppm, respectively) and has a Sr/Mn ratio of 14, suggestingdiagenetic alteration of the sample, although the δ18O value of this is inthe range of δ18O values of well-preserved specimens (Table 1, Fig. 8).Contamination by pyrite or Fe and/or Mn oxides in this sample cannotbe totally discarded, however, because the altered sample obtainedfrom the same specimen has a much higher Fe and Mn content (809and 507 ppm, respectively), a much lower Sr/Mn ratio, and lower δ18Ovalues (Table 1, Fig. 8) indicative of intense diagenetic alteration.
High Sr concentration (>950–1200 ppm) can also be indicative ofgood belemnite preservation (e.g. Veizer, 1974; Price and Sellwood,1997; Rosales et al., 2001; Wierzbowski and Joachimski, 2007, 2009).In our case, the Sr content of petrographically non-altered samplesranged between 889 and 1307 ppm (mv=1110 ppm; Table 1, Fig. 8),and most were in the range of values established by the authors citedabove. Four samples (numbers 43, 46, 76 and 78) had Sr contentunder 950 ppm. These samples also had low Fe and Mn content, Sr/Mn ratio >80 and δ18O values in the range ofwell-preserved specimens(Table 1, Fig. 8), suggesting that the lower content in Srwas not a diage-netic effect but perhaps a vital and/or environmental effect. Onlysample number 46 had high Fe content but, as discussed above, wasprobably due to contamination. The Mg concentrations (or Mg/Caratios) obtained frompetrographicallywell-preserved samples (between1745 and 6638 ppm, mv=3531 ppm, Table 1) are also in line withvalues given by other authors (e.g. Price and Sellwood, 1997; Podlahaet al., 1998; Rosales et al., 2001; McArthur et al., 2007a; Wierzbowskiand Joachimski, 2007, 2009).
Intra-specimen elemental composition heterogeneities were ana-lyzed by ICP-MS in three well-preserved specimens (PC-9-3, PC-37-4and PC-45-14, Table 1). In these three specimens the differencesobtained in the Mg and Sr contents are lower than 470 and 110 ppm,respectively. Intra-specimen elemental heterogeneities of two speci-mens (PC-31-14 and PC-37-4a) studied with the electron microprobeshowed higher differences than those obtained with the ICP in thesame specimens (Tables 1, 2). The Mg content and the S contents ofthe two belemnite specimens analyzed with the electron microprobe
Table 1Stable isotopes and elemental (ICP-MS) data of the studied belemnite specimens. The specimens are named by the section from which they were obtained followed by a number,which is representative of the stratigraphic level. Thus, those specimens, which were collected from the same stratigraphic level, have the same initial number. Asterisks are rep-resented where two thin sections (one longitudinal and the other transverse) were performed from the same belemnite. Numbers to the left are the same than those represented inFig. 8. Geochemical data obtained from petrographically altered samples are in red, those obtained from non-altered samples are in black. As in Fig. 8, the light blue horizontal rect-angles indicate those specimens from which two samples (one from petrographically altered areas and the other from non-altered areas of the same specimen) were analyzed.
varied up to 2943 and 2050 ppm, respectively (Table 2),with higherMgand S values obtained in the growth rings displaying intense FL (Fig. 9).Sr content of these two specimens also varies within a single belemniterostra (up to 1573 ppm, Table 2), however, such variations appear to beunrelated with the FL pattern (Fig. 9). The different elementalintra-specimen heterogeneities obtained with the ICP and the micro-probe can be due, as mentioned before, to the lower precision of themicroprobe compared with the ICP-MS and/or because the areas ofboth specimens analyzed with the ICP and the electron microprobewere different, or even may reflect some contribution from an organiccomponent to the electron microprobe data versus the ICP.
Inter-specimen heterogeneities in the Mg and Sr content ofwell-preserved specimens were studied in 21 stratigraphic levels(14 of the PC section and 9 of the RS section, Table 1) where more thanone specimenwas collected from the same stratigraphic level. The differ-ences among theMg and Sr values obtained fromdifferent specimens of asingle stratigraphic level range between 5 and 2962 ppm of Mg (mv=911 ppm) and between 14 and 229 ppm of Sr (mv=99 ppm). Thesevalues are within the range of values obtained by microprobe in a singlebelemnite (see above) or those given by other authors who suggest thatco-occurring belemnite genera may have diverse Mg contents if theyunderwent different metabolic fractionation (McArthur et al., 2004),though sexual dimorphism is another possibility (McArthur et al., 2007a).
Petrographically altered samples contain similar Mg content, higherFe and Mn values and lower Sr/Mn ratios than non-altered samples(Fig. 8 and Table 1), as established previously (e.g. Veizer, 1974;
Veizer and Fritz, 1976; Jones et al., 1994a, 1994b; Price and Sellwood,1997; Rosales et al., 2001; Wierzbowski, 2004; Wierzbowski andJoachimski, 2007). However, some petrographically altered sampleshave Fe, Mn contents and Sr/Mn ratios in or very close to the range ofvalues given as indicative of original marine geochemistry (e.g. samples5, 15, 18, 57, 58, 62, 75, 85 and 86; Fig. 8 and Table 1). However, theδ18O and/or δ13C values of these samples can be very different tothose obtained from well-preserved areas of the same belemnites;thus, as discussed below, caution should be taken if elemental anal-yses are used as the most significant criteria for discriminating diage-netically altered and non-altered samples, since paleoenvironmentaland/or paleotemperature interpretations based on their isotopic valuescould be erroneous.
5.2.2. Stable isotopesδ18O and δ13C values obtained from well-preserved studied belem-
nites range between −0.96 and −0.19‰, and between −1.21 and+1.24‰, respectively. Similar values are reported for well-preservedbelemnites from the Oxfordian–Kimmeridgian elsewhere in theTethys realm (Price and Sellwood, 1994; Wierzbowski, 2002,2004; Wierzbowski et al., 2009). For instance, belemnites from theKimmeridgian–Tithonian of Mallorca Island, East Spain (Price andSellwood, 1994) yield values of −0.99 to +0.04‰ in δ18O, and −2.03to +1.51‰ in δ13C; and belemnites from the Oxfordian–EarlyKimmeridgian of Poland and southern Germany (Wierzbowski, 2002,
Fig. 8. Diagrams showing geochemical data, both elemental (ICP-MS) and isotopic, obtained in petrographically altered (red) and non-altered (black) samples obtained from thebelemnite specimens analyzed (see all data in Table 1). Vertical scale (to the left) is the same for all the diagrams and numbers correspond to the specimens analyzed: purplenumbers correspond to specimens obtained from the PC section, green numbers correspond to specimens obtained in the RS section. In the first diagram to the left the Fe, Mnand Sr contents (ppm) are represented. The brown and green vertical rectangles represent the range of maximum Mn and Fe values, respectively, above from which samplesare considered as diagenetically altered by previous authors (see text). The pink vertical rectangle represents the range of Sr values, which are considered as representative oforiginal marine by previous authors (see text). Next diagram to the right represent the Sr/Mn ratios and the Mg/Ca ratios (the last in mmol/mol). Purple vertical line in this diagramrepresent the lowest Sr/Mn ratio below which samples are considered as diagenetically altered by previous authors (see text). In the next two diagrams to the right the oxygen andcarbon isotopic composition are represented. The light blue horizontal rectangles indicate those specimens from which two samples (one from petrographically altered areas andthe other from non-altered areas of the same specimen) were analyzed. Asterisks indicate the petrographically altered samples, which has Fe, Mn and Sr/Mn ratios within the rangeof values, which are considered as representative of original marine geochemistry by many authors (see text).
2004; Wierzbowski et al., 2009) yield δ18O values of −1.4 to +0.8‰,with δ13C ranging −1.8 to +2‰.
In analyzing intra-specimen heterogeneities, two different analyseswere performed in well-preserved areas of three specimens (Table 1).The differences in the isotopic values obtained between the analysesare up to 0.23‰ in oxygen and up to 0.29‰ in carbon. These data arewithin the range of variation observed by authors such as Podlahaet al. (1998) and Rosales et al. (2001), who performed spot analysesin different parts of belemnites and found differences of up to 1‰ and0.3‰ in δ18O, respectively, andof up to 1.5‰ and0.3‰ in δ13C, respective-ly and are also in the range of values reported by Dutton et al. (2007) andWierzbowski and Joachimski (2009), who performed continuous isotopicanalyses along the belemnite rostra and obtained differences of up to 1‰and 1.5‰ in δ18O, respectively, and up to 3‰ and 2.5‰ in δ13C, respec-tively. Differences in the δ18O valueswithin a single belemnitemay reflectthe variability of seawater temperature during belemnite growth, if it isassumed that belemnite migrated laterally and vertically through seawa-ter at different temperatures, and that calcite precipitated in oxygen iso-tope equilibrium with ambient seawater (Sælen et al., 1996; Price andSellwood, 1997; Rosales et al., 2001; Dutton et al., 2007; McArthur et al.,2007a; Wierzbowski and Joachimski, 2009). Intra-specimen differencesin the δ13C values have been related to the variability of the depth
at which belemnite grew, given that carbonate secreted in equilibriumwith surface waters usually has a higher δ13C value than carbonateformed at depth (Dutton et al., 2007), and as a result of ontogenetically-controlled fractionation of carbon by metabolic activity of belemnites,depending on food supply, stress conditions, growth rate, or other factors(Wierzbowski and Joachimski, 2009).
In our case, inter-specimen heterogeneities were seen in well-preserved samples from 21 different stratigraphic levels (14 from PCsection and 9 from RS section)wheremore than one specimenwas col-lected (Table 1). The differences in isotopic values ranged between0 and 0.81‰ for oxygen (mv=0.25‰) and between 0.08 and 1.23‰(mv=0.48‰) for carbon. These differences are lower than those ob-served by Rosales et al. (2001) – up to 1‰ in δ18O and up to 2.1‰in δ13C – but similar to those reported by Dutton et al. (2007)and Wierzbowski and Joachimski (2009) from a single specimen.Nevertheless, it cannot be ruled out that different belemnite species(with different isotopic compositions) were most likely analyzed inour study, as co-existing species with different paleoecological habitatsor metabolic fractionation may exhibit distinct isotopic compositions(e.g. Rosales et al., 2001; McArthur et al., 2004, 2007a).
Correlations between Mg/Ca ratios and δ18O values, and betweenthe Sr/Ca ratios and δ18O values, of well-preserved belemnite rostra
Fig. 9. Elemental geochemistry profiles performed with the electron microprobe along three belemnite specimens (the scale is given in mm within each profile). Photographscorrespond to the BFL images of each belemnite rostra after the profiles (red arrows) were performed. Vertical dotted lines are placed in the inflection points where the trendof the Mg content changes from decreasing to increasing. Note the similarity between the tendency of the Mg and the S along the profiles and the FL pattern. cps: counts per second.A. Mg, S, Sr, Fe and Mn profiles along the specimen PC-37-4a. B. Mg, S, Sr, Fe and Mn profiles along the specimen PC-45-14b. C–D. Correlative Mg, S, Sr and Fe profiles alongthe specimen PC-31-4. Blue arrows correspond to the same position along the belemnite rostrum.
Fig. 10. Cross plot showing correlation between different geochemical data. A. Mg against S concentrations obtained with the electron microprobe (see data in Table 2) from twobelemnite rostra (PC-31-41 and PC-37-4a). B. δ18O against δ13C values obtained from petrographically altered and non-altered samples. C. δ18O values against Sr concentrations (ppm).D. δ18O values against Fe concentrations (ppm). E. δ18O values againstMn concentrations (ppm). F. δ18O values againstMg/Ca ratios (mmol/mol). Note the absence of correlation betweenthe δ18O values and the rest of the geochemical parameters. Only there is some correlation between the δ18O and the δ13C values obtained from altered samples.
have been attributed to changes in the temperature and/or salinity ofseawater (e.g. McArthur et al., 2000, 2007a; Bailey et al., 2003; Rosaleset al., 2004a, 2004b), whereas the absence of such correlations hasbeen associated with diagenetic alteration (e.g. Rosales et al., 2004b).Still other authors find no or only weak correlation between the δ18O
values and Mg/Ca and Sr/Ca ratios of some belemnite genera (e.g.McArthur et al., 2004, 2007b; Dutton et al., 2007; Wierzbowski andJoachimski, 2009). These variations have been tied to ontogeneticbiofractionation effects (McArthur et al., 2007a; Wierzbowski andJoachimski, 2009). In our case, there was a lack of correlation between
Table 2Sr, Mg, S, Fe and Mn concentrations in ppm obtained from spot analyses performed withthe electron microprobe along the profiles shown in Fig. 9A (specimen PC-37-4a) andFig. 9C and D (specimen PC-31-41).
δ18O values and Mg/Ca and Sr/Ca ratios of well-preserved belemnites,mainly corresponding to the genus Hibolithes (Fig. 10C, F); this couldbe related to biofractionation, as previous geochemical studies ofsome species of this genus have shown (e.g. McArthur et al., 2004;Wierzbowski and Joachimski, 2009). Nevertheless, the very smalldifference between the maximum and the minimum δ18O values alongthe studied stratigraphic sections (1.15‰, Fig. 8, Table 1), is noteworthyif compared with the δ18O variations over 3‰ reported by other authors(e.g. Rosales et al., 2001, 2004b; Bailey et al., 2003;McArthur et al., 2007a,2007b). These authors also report good correlation between δ18O valuesand Mg/Ca and/or Sr/Ca ratios, and suggest that changes in these values
may reflect conditions of seawater temperature and/or salinity. Accord-ingly, the lack of correlation we observed may indicate only minorvariations in temperature and/or salinity, scarcely affecting Mg and Srconcentrations.
Petrographically altered samples have, in general, more negativeδ18O values than well-preserved samples. Moreover, δ18O values ofaltered samples covary with the δ13C values (Fig. 10B), suggestinggeochemical alteration of belemnite calcite by diagenetic fluids duringprogressive burial (e.g. Lohmann, 1988; James and Choquette, 1990;Marshall, 1992; Jenkyns et al., 1994; Sælen et al., 1996; Veizer et al.,1997; Rosales et al., 2001). Although δ13C values of altered portionsof the belemnites may be more negative or more positive than thenon-altered portions of the same belemnite, the range of δ13C valuesof non-altered and altered samples is almost identical (Figs. 8, 10,Table 1), suggesting that δ13C values are not always indicative of diage-netic alteration. Likewise, the lack of correlation of Sr, Fe and Mn con-tents with δ18O, which has been interpreted as an indication ofminimal postdepositional alteration (Rosales et al., 2001), is not reliablefor discriminating diagenetic alteration because in our case both petro-graphically non-altered and altered portions of the belemnites show nocorrelation between these parameters (Fig. 10C–E).
Another important issue is the isotopic composition of the petro-graphically altered samples that have Fe, Mn and Sr/Mn contentsinside or very close to the range of values given as indicative of orig-inal marine geochemistry (numbers 5, 15, 18, 57, 58, 62, 75, 85 and86, Fig. 8). The δ18O values of samples 5, 57 and 62 are almost identi-cal to those obtained from petrographically non-altered parts of thesame specimen (Fig. 8), even though their Fe and Mn contents arehigher, and the Sr/Mn ratios are lower than those obtained fromnon-altered areas. However, the δ18O values of the altered portionsof other such specimens (samples 15 and 58) are muchmore negativethan those obtained from non-altered areas of the same specimen(for example a difference of −1.84‰ for sample number 15; Fig. 8).Isotopic data of the other four petrographically altered samples,which have Fe, Mn and Sr/Mn contents similar to the original marinevalues (samples 18, 75, 85 and 86; Fig. 8) cannot be compared withthose obtained from non-altered areas. Nonetheless, the δ18O valuesof sample numbers 18, 75 and 86 are more negative than any δ18Ovalue obtained from any petrographically non-altered sample (from−0.6 to −1‰; Fig. 8) suggesting diagenetic alteration and the δ18Ovalue of specimen number 85 is in the range of the δ18O valuesobtained from non-altered samples, although it is more negativethan most of them (Fig. 8). Thus, if these samples had been taken asnon-altered samples (based on their elemental chemistry) and suit-able for performing isotope analyses, the temperatures obtainedfrom the δ18O values of these altered areas would have been errone-ous, and up to 8 °C warmer if the Friedman and O'Neil (1977) formulawas applied. The δ13C values of these samples that, being petrograph-ically altered, have Fe, Mn and Sr/Mn contents included in or veryclose to the range of values given as original marine (samples number5, 15, 18, 57, 58, 62, 75, 85 and 86) show no overall trend, and all lie inthe range of δ13C values obtained from non-altered samples (Fig. 8).Moreover, the δ13C values of the petrographically altered portionsof the samples, which can be compared to those obtained from thenon-altered portions of the same specimens (numbers 5, 15, 57, 58 and62) likewise show no trend and they may be more negative or morepositive than the δ13C values obtained from non-altered portions inde-pendently of the offset that was observed in the δ18O values (Fig. 8).
Thus, the geochemistry of diagenetically altered and non-alteredportions of belemnite rostra performed in this study suggests that cau-tion should be taken if elemental analyses are used as the most signifi-cant criteria for discriminating diagenetically altered and non-alteredbelemnite samples, because paleoenvironmental or temperature inter-pretations based on their isotopic values could be largely erroneous. Onthe contrary, our study suggests that elemental and isotopic analysesshould be performed in samples obtained directly from the thin
sections previously studied petrographically, which allows for fullcontrol of the areas to be sampled and would minimize samplingcontamination, while making it possible to sample belemnite speci-mens that are partly diagenetically altered.
5.3. Belemnite taphonomy and facies
Unlike biostratinomic features (see Part I, Reolid and Benito,2012-this issue), fossil–diagenetic ones are not clearly related tolithofacies — diagenetic alteration is similar in both sections and inthe different lithofacies. The only features apparently distinctive area higher degree of diagenetic recrystallization in samples obtainedfrom the PC section (and mainly from the spongiolithic and marly fa-cies) and a lower degree of recrystallization and a higher degree offracturing of samples from the RS section, probably due to its lowersedimentation rate. Moreover, the brownish, translucent appearanceand fetid smell of the RS belemnites suggest greater preservation oforganic matter in this section.
6. Conclusions
Conclusions from the fossil–diagenetic features (both petrographicand geochemical) of belemnites from a Late Jurassic epicontinentalplatform of southern Spain (Betic Cordillera) can be summarized asfollows:
1.- The most common fossil–diagenetic processes are dissolution,calcite cementation and recrystallization, mainly of the apicalzone and outer growth rings of belemnites. These processes mayhave been enhanced by fracturing and stylolite formation.
2.- Diagenetic alteration of belemnites started during the earliest stagesof diagenesis, previous to belemnite burial (as a biostratinomic pro-cess), then continuedduring progressive burial (as a fossil–diageneticprocess).
3.- Despite the diagenetic alteration of some growth areas, an originalconcentric growth pattern was observed under epifluorescencemicroscopy and BSEM. This growth pattern is also revealed bychanges in the Mg and S contents of the rostra.
4.- Detailed geochemical study shows that caution should be taken ifelemental analyses are used exclusively or as the most significantcriterion for discriminating diagenetically altered and non-alteredbelemnite samples. On the contrary, our findings suggest thatelemental and isotopic analyses should be performed in samplesobtained directly from thin sections previously studied petrographi-cally, through a combination of TL, CL and FL, which ensures controlof the areas to be sampled and minimizes sampling contamination.
5.- Fossil–diagenetic processes typically make specimens unsuitablefor paleoenvironmental interpretations. However, microsamplingof petrographically altered and non-altered areas from the samespecimens, performed separately and directly from thin sectionsafter the petrographic study, made it possible for us to obtain excel-lent and reliable results.
Acknowledgments
This research was carried out with the financial support of theprojects RYC-2009-04316 (Ramón y Cajal Program, Ministerio de Cienciae Innovación), CGL2009-10329, CGL2011-22709 andCGL2008-03007/CLI(Ministerio de Ciencia e Innovación) and the Paleoclimate ResearchGroup of the University Complutense of Madrid. We are grateful to thereviewers. We also thank a native English speaker (Jean Louise Sanders)for reviewing the grammar and Antonio Piedra (Technician ofDepartamento de Geología, Universidad de Jaén) for preparing the thinsections.
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